1. Field of the Invention
The present invention relates to a focus detection apparatus and, more particularly, to a focus detection apparatus used in a single-lens reflex camera and capable of appropriately performing focus detection even if an object to be photographed has a low brightness level.
2. Related Background Art
As a conventional focus detection apparatus for a camera or the like, there is known an apparatus wherein an object image is photoelectrically converted using a photoelectric converting means such as a CCD element to obtain an object image signal, and focus detection is performed on the basis of the object image signal.
FIG. 19 shows a structure of part of a CCD element as a photoelectric converting means used in the above focus detection apparatus. The CCD element shown in FIG. 19 comprises a photodiode section PD for generating a charge upon photoelectric conversion, a barrier gate section BG, a storage section ST, a transfer gate section TG, a charge transfer register section CCD, a clear gate section CG, and an overflow drain section OFD. Of these sections, the photodiode section PD generates a charge corresponding to the amount of incident light upon photoelectric conversion. The barrier gate section BG receives a fixed voltage V1 to constitute a barrier between the photodiode section PD and the storage section ST. The charge generated by the photodiode section PD exceeds a potential of the fixed voltage V1 and flows in the storage section ST. The storage section ST receives a fixed voltage V2 and constitutes a potential well for storing the charge. The transfer gate TG is controlled in response to a transfer pulse .phi.T and transfers the charge stored in the storage section ST to the register section CCD. The clear gate section CG discharges an unnecessary charge of the storage section ST to the overflow drain section OFD in response to a clear pulse .phi.C.
Referring to FIG. 19, the photodiode section PD, the barrier gate section BG, the storage section ST, the clear gate section CG, the overflow drain section OFD, and the transfer gate section TG are arranged in units of pixels. A plurality of pixels are linearly arranged along the register section CCD.
FIG. 20 shows potentials of the respective sections when a charge generated by the photodiode section PD is stored in the charge storage section ST. That is, in this case, the charge generated by the photodiode section PD constituting each pixel is transferred to the storage section ST through the barrier gate section BG. Since the potentials of the transfer gate section TG and the clear gate section CG are high, the charge is stored in the storage section ST.
FIG. 21 shows potentials of the respective sections when the charge stored in the storage section ST is transferred to the register section CCD. In this case, after the charge is stored in the storage section ST, the potential of the transfer gate section TG is decreased in response to the transfer pulse .phi.T, and the charge stored in the storage section ST is transferred to the register section CCD.
The charge stored in the register section CCD is sequentially transferred in response to two phase clocks .phi.1 and .phi.2.
FIG. 22 shows potentials of the respective sections when the charge of the photodiode section PD is not stored. That is, in this case, the charge generated by the photodiode section PD is transferred to the storage section ST through the barrier gate section BG. On the other hand, since the potential of the clear gate section CG is set lower than that of the transfer gate section TG in response to the clear pulse .phi.C, the charge input to the storage section ST is discharged to the overflow drain section OFD through the clear gate section CG.
In the photoelectric conversion element described above, the pixels are aligned along the register section CCD to constitute a line sensor, and a signal corresponding to a light amount distribution of the object is output to detect a focusing state.
In the photoelectric converting means described above, when the object brightness level is low, an output from the photoelectric conversion element is lowered, and at the same time, the signal-to-noise ratio is also lowered. As a result, it is difficult to perform focus detection.
In order to solve this problem, the area of the photodiode section PD of the photoelectric conversion element may be increased to increase a charge amount. Since the dimension of the photodiode section PD in the alignment direction is associated with the sample pitch of the object image and has an influence on the focus detection precision, the dimension of the photodiode section PD cannot be unlimitedly increased in this direction.
When the dimension of the photodiode section PD in a direction perpendicular to the alignment direction thereof is increased, a distance from the end of the photodiode section PD to the storage section ST is increased to undesirably prolong the charge transfer time. Charge storage control at a high brightness level becomes difficult, or charge transfer becomes imperfect to form an afterimage by a residual charge.